26.6.3 Emacs Lisp Documentation Lookup

When editing Emacs Lisp code, you can use the commands C-h f
(describe-function) and C-h v (describe-variable)
to view the built-in documentation for the Lisp functions and
variables that you want to use. See Name Help.

Eldoc is a buffer-local minor mode that helps with looking up Lisp
documentation. When it is enabled, the echo area displays some useful
information whenever there is a Lisp function or variable at point;
for a function, it shows the argument list, and for a variable it
shows the first line of the variable's documentation string. To
toggle Eldoc mode, type M-x eldoc-mode. There's also a Global
Eldoc mode, which is turned on by default, and affects buffers, such
as ‘*scratch*’, whose major mode is Emacs Lisp or Lisp
Interaction (M-x global-eldoc-mode to turn it off globally).